Exhaust system members such as exhaust manifolds, exhaust pipes, converter cases, and mufflers, used in exhaust environments of automobiles are required to have superior formability and superior heat resistance. Conventionally in many cases, Cr-containing steel sheets containing Nb and Si, for example, Type 429 (14Cr-0.9Si-0.4Nb-base) steel, which is malleable, has superior formability at room temperature, and has relatively increased high-temperature strength, have been used for the aforementioned applications.
However, when exhaust gas temperatures are increased to 900° C. to 1000° C., which is higher than can be endured, due to improvements in engine performance, there is a problem in that Type 429 steel has insufficient high-temperature proof stress or oxidation resistance.
Accordingly, a material having strength higher than that of Type 429 steel at 900° C. and having superior oxidation resistance is required. When the high-temperature strength of the material for the exhaust system members is increased, it becomes possible to reduce the thicknesses of the members so as to advantageously contribute to reduced weight of automobile bodies.
For example, in Japanese Unexamined Patent Application Publication No. 2000-73147, a Cr-containing steel having superior high-temperature strength, formability, and surface properties is disclosed as a material which can be applied to a wide range of temperatures from the high temperature portion to the low temperature portion of the exhaust system member. This material is a Cr-containing steel containing C: 0.02 mass percent or less, Si: 0.10 mass percent or less, Cr: 3.0 to 20 mass percent, and Nb: 0.2 to 1.0 mass percent. By decreasing the Si content to 0.10 mass percent or less, precipitation of the Fe2Nb Laves phase is suppressed in order to prevent an increase in yield strength at room temperature, and to be invested superior high-temperature strength and formability, as well as excellent surface properties.
European Patent Application Publication No. EP1207214 A2 discloses that precipitation of the Laves phase is suppressed to ensure that strength at high temperature is stably increased in solid solution Mo under the conditions that satisfy C: from 0.001% to less than 0.020%, Si: more than 0.10% to less than 0.50%, Mn: less than 2.00%, P: less than 0.060%, S: less than 0.008%, Cr: 12.0% or more to less than 16.0%, Ni: 0.05 or more to less than 1.00%, N: less than 0.020%, Nb: 10×(C+N) or more to less than 1.00%, Mo: more than 0.8% to less than 3.0%; wherein Si≦1.0-0.4 Mo, and W: 0.50% or more to 5.00% or less, as required.
These two publications aim to improve the high-temperature strength at 900° C. The strength and the oxidation resistance at 900° C. are evaluated in the these art.
However, the above-mentioned material for exhaust members still have problems in terms of the oxidation resistance at high temperature, i.e., 900° C. to 1000° C.
In order to improve engine performance, a significant increase in the exhaust gas temperatures is unavoidable. When the exhaust temperature is increased to 900° C. to 1000° C., the conventional material exhibits extraordinary oxidation, or has poor high-temperature strength.
The term “extraordinary oxidation” herein refers to the phenomenon that the material becomes ragged. When the material is exposed to the high temperature exhaust gas, a Fe oxide is produced, which is extremely rapidly oxidized.
The term “salt corrosion at high temperature” herein means that the sheet thickness becomes thinner due to corrosion. The corrosion occurs when salts in an antifreezing agent applied on road surfaces in cold regions, or salts in seawater near shores become attached to the exhaust pipes and then are heated at high temperature. It could therefore be advantageous to provide a ferritic Stainless steel which has excellent strength at high temperature, oxidation resistance at high temperature, and salt corrosion resistance at high temperature.